The present invention relates to a scintillation camera capable of correcting errors in detected radiation position to provide a tomogram of high accuracy, said errors in detected radiation position being caused due to the non-linearity detection characteristic of radiation detector.
There have been conventionally used scintillation cameras wherein radiation or gamma ray radiated from human body or the like into which radio-isotope was injected is detected to provide a tomogram necessary to make a diagnosis relating to a desired portion of human body. The radiation detecting device incorporated into the scintillation camera an inherent non-linear detection characteristic and the tomogram detected comes to have distortion unless non-linearity detection characteristic is corrected.
There is known in the art a scintillation camera capable of effectively correcting non-linearity detection characteristic. This known scintillation camera comprises a radiation detector for detecting radiation or gamma ray, a position calculating circuit for calculating, responsive to outputs applied from the radiation detector, radiation emitting positions which will be hereinafter referred to as radiation positions or positions, an A/D converter for converting outputs or position signals to digital signals, position signals being applied from the position calculating circuit, a memory for storing a reference correcting amount for each of a plurality of lattice points of a given matrix optionally determined on an X-Y plane, the reference correcting amounts being used to correct position signals, which have been converted to digital signals by the A/D converter, to accurate position signals representing accurate positions and the position signals already including error components due to non-linear detection characteristic inherent in the radiation detector and representing no correct positions or accurate positions on X-Y plane corresponding to actual positions, a correcting amount calculation circuit for reading out reference correcting amounts in four cross points in the memory adjacent to calculated positions so as to correct calculated radiation positions, which have been calculated by the position calculating circuit and digitalized by the A/D converter, to accurate positions in a measuring mode and for calculating correcting amounts to correct calculated positions to accurate positions, the four cross points containing calculated positions in an area formed by connecting four cross points with one another by straight lines, an adder for adding the calculated correcting amount to the calculated position, and a display device for displaying resultant positions to visualize a tomogram relating to a desired portion of human body.
Since the scintillation camera of this type carries out correction of non-linearity of position calculation circuit in a data processing manner, adjustment relating to variation per hour and the like can be relatively easily achieved and correction of high accuracy can also be attained without degrading resolving-power, quality of picture and the like.
Operation in the correcting amount calculation circuit is carried out as follows: It is assumed that a calculated radiation position P(Xp,Yp) converted by the A/D converter to a digital value is contained, as shown in FIG. 1, in an area enclosed by four cross points A(X.sub.i,Y.sub.j), B(X.sub.i,Y.sub.j+1), C(X.sub.i+1,Y.sub.j+1) and D(X.sub.i+1,Y.sub.j) of a given matrix in the memory. In this case, reference correcting amounts (X.sub.i,j ; Y.sub.i,j), (X.sub.i,j+1 ; Y.sub.i,j+1), (X.sub.i+1,j+1 ; Y.sub.i+1,j+1) and (X.sub.i+1,j ; Y.sub.i+1,j) at four cross points A, B, C and D are read out of memory by the correcting amount calculation circuit.
Reference correcting amount (X.sub.i,j ; Y.sub.i,j) will be explained with reference to FIG. 2, X.sub.i,j represents an X component of correcting vector and Y.sub.i,j a Y component thereof. The same is true of other reference correcting amounts. It is now assumed that corrected accurate positions of four lattice points A, B, C and D and A', B', C' and D' and that cross points formed by drawing vertical lines from position P(Xp,Yp) toward sides AB, BC, CD and DA, respectively, are Q, R, S and T. The following relations expressed by equations (1), (2), (3) and (4) are established in this case: EQU BQ: QA=B'Q': Q'A' (1) EQU BR: RC=B'R': R'C' (2) EQU Cs: Sd=C'S': S'D' (3) EQU DT: TA=D'T': T'A' (4)
A cross point P' between lines R'T'and Q'S' can be obtained from above-mentioned relations. Vector PP' now becomes a correcting vector corresponding to a correcting amount in relation to position P.
When correction of non-linearity is carried out like this in the scintillation camera, correction of higher accuracy can be achieved as quantizing bit number of A/D converter becomes larger and larger.
However, as quantizing bit number of A/D converter becomes larger and larger, the A/D converter becomes complicated in arrangement, high in cost and slow in converting speed. It is therefore desirable that an A/D converter having as few a quantizing bit number as possible but so few a quantizing bit number as to have no influence to correction accuracy is employed.
When theoretical consideration is paid to resolving-power, it is unnecessary that the A/D converter has a large quantizing bit number and the quantizing bit number may be such that it is obtained by applying the sampling theorem to space frequency corresponding to the resolving-power determined by the device itself.
When the quantizing bit number of the A/D converter becomes small, however, resolving-power is not lowered theoretically, but the following practical problem is caused.
Namely, it is assumed that M1, M2, M3, . . . shown in FIG. 3 are unit meshes for quantizing position signals by the A/D converter. All points contained in each of unit meshes M1, M2, M3, . . . are collected and recognized as typical points MP1, MP2, MP3, . . . by A/D conversion of A/D converter. Correcting vectors CV1, CV2, CV3, . . . corresponding to typical points MP1, MP2, MP3, . . . are calculated by the correcting amount calculation circuit. Since these correcting vectors CV1, CV2, CV3, . . . are obtained on the basis of MP1, MP2, MP3, . . . which are typical points of unit meshes M1, M2, M3, . . . they are applied to all points contained in unit meshes M1, M2, M3, . . . Therefore, all of positions corrected on the basis of correcting vector CV1 are contained in a unit region M1' primarily determined by correcting vector CV1. All of positions corrected on the basis of correcting vector CV2 are included in a unit region M2' primarily determined by correcting vector CV2. Similarly, all of positions corrected on the basis of correcting vector CV3 are included in a region M3' primarily determined by correcting vector CV3. As described above, unit regions M1', M2', M3', . . . in which corrected positions are included are limited to those corresponding to unit regions M1, M2, M3, . . . and positions of unit regions M1', M2', M3', . . . themselves are limited by correcting vectors corresponding to typical points MP1, MP2, MP3, . . . of unit meshes. Therefore, in regions where correcting vectors adjacent to each other intend to become more remote from each other as compared with them before correction, an area which is not covered by unit regions is caused as shown by unit regions M1' and M2'. Namely, a region where non-corrected position is present is caused. In regions where correcting vectors adjacent to each other intend to become nearer as compared with them before correction, an area where unit regions are overlapped each other is caused as shown by unit regions M1' and M3'. Namely, a region where corrected positions are overlapped one another is caused.
The accuracy of a tomogram obtained is lowered due to irregular correction like this. However, it is not desirable that an A/D converter having a larger quantizing bit number than needed in the viewpoint of resolving-power is employed only for the purpose of eliminating errors caused due to irregular correction.